The invention relates generally to precision measurement instruments, and more particularly to a system and method for dynamic calibration of quadrature signals such as those used in a position encoder.
Various position encoders for sensing linear, rotary or angular movement are currently available. These encoders are generally based on inductive, capacitive, optical, or magnetic transducers. In general, an encoder may comprise a transducer with a readhead and a scale. The readhead may comprise a transducer element and some transducer electronics. The transducer output signals vary as a function of the position of the readhead relative to the scale along a measuring axis. The transducer electronics output the signals to a signal processor or process the signals internally before outputting modified signals indicative of the position of the readhead relative to the scale. It is also common for an encoder system to include an interface electronics separate from the readhead, and to interpolate or otherwise processes the transducer signals in the interface electronics before outputting modified signals indicative of the position of the readhead relative to the scale to an external host system such as a motion control system or data acquisition system.
Certain encoder systems utilize quadrature signals. As an example, in a two-phase system a transducer may output two signals A and B that vary sinusoidally as a function of the position of the readhead relative to the scale along the measuring axis. In one common concept for transducers, the signals A and B are intended to be identical except for a quarter-wavelength phase difference between them. In other words, the signals are in quadrature (i.e., a quarter-wavelength out of phase relative to each other.) In other systems (e.g., 3-phase), the signals may be combined so as to produce similar signals in quadrature. The transducer electronics then use these quadrature signals to derive the instantaneous position of the readhead relative to the scale along the measuring axis.
For encoders that utilize quadrature signals, three common error sources that may occur are signal offsets, amplitude mismatches and phase errors. U.S. Pat. No. 6,897,435, hereby incorporated herein by reference in its entirety, discloses a self-calibration method that is intended to address these types of error sources for quadrature encoders. As described in the '435 patent, the encoder includes a circuit for generating gain, offset, and phase calibration coefficients, where the circuit compares the phase space position of a measured phasor with the position of an idealized phasor. The locus of the idealized phasor in phase space is intended to be a circle of predetermined radius with no offset. The encoder applies the calibration coefficients to the measured quadrature signals, and creates an output signal representative of the current, calibrated phase. For generating the calibration coefficients, a coefficient generator module applies a series of logical tests to decide if the measured phasor lies on a unit circle. If the measured phasor is not on the unit circle, then the module increments or decrements the various calibration coefficients until the phasor does lie on that circle.
The '435 patent describes an example where the measured phasor has only a positive offset. In a first position, where the measured phasor is at approximately 0°, the gain and offset are incremented in the negative direction, while at a second position when the measured phasor is at approximately 180°, the offset is again incremented in the negative direction but the gain is incremented in the positive direction. It is noted that in this example, the gain is alternately reduced and increased, with a net effect of no change, while the offset is continually made more negative, which thus corrects for the initial positive offset. This example illustrates one of the drawbacks of the method of the '435 patent, in that while the net result to one of the calibration coefficients (i.e., the gain) is no change, the process of incrementing and decrementing the calibration coefficients in this manner, and the general concept of making such changes based only on the data of the current measured phasor position, may in some implementations be considered inefficient.
The present invention is directed to a quadrature encoder that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to a system and method for dynamic calibration, wherein the calibration coefficients are determined by storing a history of sample values for which a specified criteria (e.g., the variance of a metric related to the sample values) is utilized for the determination of the optimum calibration coefficients.